The present invention relates to differential amplifiers using n-channel insulated-gate field-effect transistors, in which the one end of the source-drain section of each amplifier transistor of the enhancement type, each by way of a load transistor of the depletion type whose gate is connected to the source thereof, is applied to the voltage-conducting pole of a source of operating voltage, while the other end, by way of a common constant-current circuit using at least one constant-current transistor of the enhancement type, is connected to the zero point of the circuit (ground), cf, the preamble of the patent claim.
A differential amplifier of this type is disclosed in the earlier published European Patent Application EP-A No. 0 139 078, especially in connection with FIG. 1 thereof. Since depletion-made transistors are used as load resistors of the amplifier transistors, the distortions of this amplifier are mainly determined by these depletion-mode transistors, i.e. in two different ways, namely on one hand by the chiefly square current-voltage characteristic of the load transistor and, on the other hand, by the dependence of the gate-threshold voltage upon the source-drain voltage. In the case of a monolithic integration it would be possible to provide polysilicon resistors instead of the depletion-mode transistors. These, however, would take up too much space on the crystal surface of the integrated circuit and, moreover, would also be subjected to considerable fabrication process variations.